Radiation imager with single passivation dielectric for transistor and diode

ABSTRACT

A radiation imager includes a photosensor array having a plurality of individually addressable pixels, each pixel having a photosensor island and an associated thin film transistor (TFT) disposed to selectively electrically couple the photosensor island to a predetermined address line. In each pixel a single common passivation layer is disposed over the TFT and the photosensor island such that the passivation layer is adjacent to both the outer surfaces of the TFT and portions of the photosensor island. In a method of fabricating a photosensor array as described above, after depositon of a source-drain metal layer, the layer is left unpatterned until after the photosensor island has been formed. In the formation of the photosensor island the source-drain metal layer serves as an etch stop to protect the TFT. Following formation of the photosensor island, the source-drain metal layer is patterned to form source and drain electrodes and fabrication of the TFT is completed. The single common passivation layer is then deposited over both the TFT and the photosensor island.

BACKGROUND OF THE INVENTION

This application relates to solid state radiation imagers and inparticular to a common passivation barrier for transistor and photodiodecomponents and the method of fabricating such an imager.

Solid state imagers typically include a photosensor array coupled to ascintillating medium. Radiation absorbed in the scintillator generatesoptical photons which in turn pass into a photosensor, such as aphotodiode, in which the optical photon is absorbed and an electricalsignal corresponding to the incident optical photon flux is generated.Substantially hydrogenated amorphous silicon (a-Si) is commonly used inthe fabrication of photosensors due to the advantageous photoelectriccharacteristics of a-Si and the relative ease of fabricating suchdevices. In particular, photosensitive elements, such as photodiodes,can be formed in connection with necessary control or switchingelements, such as thin film transistors (TFT's), in a relatively largearray.

Imager and display arrays are typically fabricated on large substrateson which many components, including TFTs, address lines, and devicessuch as photosensors, am formed through the deposition and patterning oflayers of conductive, semiconductive, and insulative materials. Thearray is comprised of pixels, with the address lines and associated TFTsbeing coupled together to enable the photosensor in each pixel of thearray to be respectively addressed, so that, for example, the chargedeveloped by each photosensor can be selectively read. The TFTfabrication process involves several patterning steps to produce thedesired arrangement of a channel region between a source and a drainelectrode with the desired arrangement of semiconductive materialdisposed between the electrodes and over the gate electrode. The TFT iselectrically coupled to a respective photosensor, such as a photodiode,which is disposed to absorb incident photons and accumulate theresulting charge produced in the diode.

The fabrication process for such an imaging array typically includessteps to fabricate the TFT, including forming source and drainelectrodes and associated contact pads to address lines and thephotosensor. The TFT and related assemblies are then coated with aprotective passivation coating, which must then be patterned to providevia openings for electrical contact between the photosensor body and anunderlying photosensor electrode (or contact pad, which couples thephotosensor to the TFT. The a-Si for the photodiode body is thendeposited and patterned; the presence of the protective passivationlayer over the TFT assemblies is necessary to ensure that a-Si portionsof the TFT are not damaged by the patterning process for the photosensorisland (as the etchants used to form the a-Si body would similarly etchTFT components if allowed to come in contact with such a-Si portions).

As the conventional fabrication process involves the deposition of aseparate passivation layer over the TFT before fabrication of thephotosensor body, a portion of the passivation material abuts thephotosensor contact pad in the form of a "lip" or build up of the TFTprotective passivation layer material over the edges of the photosensorisland bottom contact pad. This buildup of passivation material resultsin the subsequently deposited a-Si conforming to the shape of this lip,resulting in the lip portion of the passivation layer being disposedbetween the overlying a-Si material and the photosensor island bottomcontact pad. It has been observed, however, that such an arrangementresults in increased photodiode leakage, especially when the thicknessof the passivation material between the a-Si of the photosensor islandand the bottom contact pad is thicker than about 0.5 μm.

In the conventional array formation process, after formation of thephotosensor island another passivation layer is deposited over the arrayto provide electrical insulation between the photosensor island (exceptat a predetermined contact area) and the common electrode of thephotosensor array, which is disposed over the passivation layer.Further, the passivation layer over the photosensor protects the a-Siphotosensor from environmental conditions (such as moisture) that candegrade its performance and also protects the array from exposure tomaterials, such as solvents, used in remaining steps of the fabricationprocess. Typically a scintillator is then deposited over the commonelectrode of the photosensor array to complete the imager waferstructure.

An object of this invention is to provide a method of fabricating animager array having fewer steps and resulting in the elimination of onedielectric layer in the assembled array.

It is a further object of this invention to provide an imager having astructure with a single common passivation layer for the TFT andphotosensor in each pixel of the array.

A still further object of the invention is to provide an imager having aphotodiode structure conducive to low leakage operation of thephotodiode.

SUMMARY OF THE INVENTION

A solid state radiation imager in accordance with this inventionincludes a photosensor array disposed on a substrate, the photosensorarray comprising a plurality of individually-addressable pixels. Each ofthe pixels comprises a photosensor island of photosensitive materialdisposed on a contact electrode; a thin film transistor (TFT)electrically coupled to the photosensor contact electrode and torespective scan and data lines so that the TFT is disposed toselectively couple the photosensor island to the data line; and, asingle common passivation layer disposed over the TFT and photosensorisland. The single common passivation layer is disposed so that it isadjacent to the outer surface of the TFT and at least portions of theouter surface of the photosensor island. The single common passivationlayer comprises either a single tier or multiple tiers (or strata) ofdielectric material. The photosensor island typically comprisesamorphous silicon (a-Si) that is disposed over the photosensor islandbottom contact and substantially no portion of the passivation layer isdisposed between the a-Si photosensor body and the bottom contact pad.

A method of fabricating a radiation imager having a plurality ofindividually-addressable pixels includes the steps, for each pixel, offorming a TFT body on a substrate up to the point of depositing asource-drain metal layer, depositing a source-drain metal layer over theTFT body and remaining portions of the pixel area, forming a photosensorisland on the source-drain metal layer such that the photosensor islandoccupies portions of the pixel other than where the respective TFT isdisposed, and patterning the source-drain metal layer so as to formrespective TFT source and drain electrodes and associated contacts tothe photosensor island and predetermined address lines. The source-drainmetal layer serves as an etch protectant in the formation process of thephotosensor island such that the a-Si etchants used in formation of thephotosensor island do not etch the TFT body. Source-drain metal layercomprises two layers of metal, or alternatively, a single metal layer.Etch protection of the TFT body is provided by the etchant used informing the a-Si photosensor island being selective to the materialcomprising the upper surface of the source-drain metal layer, oralternatively, depositing the source-drain metal layer to a thicknesssuch that not more than about 90% of the thickness of the layer isremoved in the etching of the a-Si photosensor island with an etchantthat is not selective to the source-drain metal.

BRIEF DESCRIPTION OF THE DRAWINGS

The features of the invention believed to be novel are set forth withparticularity in the appended claims. The invention itself, however,both as to organization and method of operation, together with furtherobjects and advantages thereof, may best be understood by reference tothe following description in conjunction with the accompanying drawingsin which like characters represent like parts throughout the drawings,and in which:

FIG. 1 is a cross-sectional view of a portion of a pixel of a radiationimager at one stage in the fabrication process in accordance with thisinvention.

FIG. 2 is a cross-sectional view of a portion of a pixel of a radiationimager at a later stage in the fabrication process in accordance withthis invention.

FIG. 3 is a further cross-sectional view of a portion of a pixel of aradiation imager at the stage in the fabrication process in accordancewith this invention when the photosensor array has been completed.

DETAILED DESCRIPTION OF THE INVENTION

A radiation imager 100 fabricated in accordance with this inventioncomprises a plurality of pixels 110, a representative one of which isillustrated in FIG. 1. Imager, as used herein, refers to a solid statedevice that is adapted to absorb incident radiation of a particularwavelength and generate an electrical signal corresponding to theabsorbed radiation. As each pixel 110 is individually addressable, thespatial distribution of radiation absorbed by the array is alsodeterminable. Typically the imager is electrically coupled to electricalcircuits not situated on the wafer that amplify and process theelectrical signals generated by the array.

For purposes of illustration and not limitation, a representative pixel110 is illustrated in the Figures herein and the accompanyingdescription refers to that representative pixel; typically, inaccordance with known fabrication processes, fabrication of all pixelson substrate 105 that will form a particular imager wafer proceedssimultaneously.

Imager 100 comprises a photosensor array 120 disposed on a substrate105. In the fabrication of imager 100, in each pixel 110, a TFT body 130is formed on substrate 105 in accordance with processes known in theart. "TFT body", as used herein, refers generally to the materialsdisposed on substrate 105 at different stages in the fabrication processthat will comprise the TFT in the assembled array; thus, dependent onthe stage of the fabrication process, TFT body 130 does not necessarilycomprise all components necessary for a functioning TFT, although whenthe fabrication process is completed TFT body 130 will comprise anoperable TFT (which will similarly be referred to as TFT 130). At thestage of the assembly process illustrated in FIG. 1, TFT body 130comprises a gate electrode 132 (typically comprising a conductivematerial such as chromium, titanium, molybdenum, aluminum, or the like);a gate dielectric layer 134 disposed thereover (and typically comprisingan inorganic dielectric material such as silicon nitride or silicondioxide); a semiconductor material layer 136 disposed over gatedielectric layer 134 (semiconductor material layer 136 typicallycomprising amorphous silicon (a-Si)); and a doped semiconductor materiallayer 138 (typically comprising a-Si doped to exhibit n+conductivity)disposed over semiconductor material layer 136. If desired forprotection of the doped semiconductive material, a relatively thinprotective layer (not shown) is deposited over doped semiconductivematerial layer 138 prior to patterning, in accordance with knownprocedures, such as photolithography, semiconductor material layer 136and doped semiconductor material layer 138 to form a portion TFT body130 disposed over gate electrode 132. The relatively thin protectivelayer over doped semiconductive layer 136 typically comprises a metalselected to comprise the same metal as the immediate layer of thesource-drain metal that is deposited thereover, as described below.After deposition of the overlying source-drain metal, the protectivelayer over the doped semiconductive material comprises part of thesource-drain metal layer and is not readily distinguishable from theimmediately adjacent source-drain metal.

Source-drain metal layer 140 is then deposited over TFT body 130 andremaining portions of the pixel 110. In accordance with this invention,source-drain metal layer 140 comprises an etch protectant for TFT body130; as used herein "etch protectant" refers to a layer thatsubstantially prevents contact between TFT body 130 and etchants used information, as described below, of an a-Si photosensor island 150 on thesurface of source-drain metal layer 140. In one embodiment of thepresent invention, source-drain metal comprises a conductive materialselected to exhibit the desired characteristics as an etch stop infabrication steps of photosensor island 150. For example, the a-Siphotosensor islands are typically formed by etching the a-Si in plasmagasses comprising fluorine or chlorine (such as carbon tetrafluoride(CF₄); sulfur hexafluoride (SF₆); or hydrogen chloride gas (HCl)). It isthus desirable that the etchant used in forming the photosensor beselective to the conductive material comprising the upper surface ofsource-drain metal layer 140 ("upper" or "outer" surface as used hereinrefers to the surface of the layer furthermost from substrate 105; norestriction as to orientation of the device in fabrication or operationis implied). Alternatively, source-drain metal layer 140 comprises amaterial that is susceptible to etching by the etchant used in formationof a-Si photosensor island 150 but has a thickness of a magnitude suchthat not greater than about 90% of source-drain metal layer 140 isetched away in the etching process to form a-Si photosensor island 150.Other considerations in the selection of source-drain metal layer 140include resistivities of the conductive material.

In one embodiment, the source-drain metal is selected to be used both toform the source-drain electrodes for TFT body 130 and also to form adata line 145 (data line 145 is one of the address lines to which TFT130 is electrically coupled, the other address line being a scan line(not shown) to which gate electrode 132 is electrically coupled). Dataline 145 as shown in FIG. 2 would extend out of the plane of the figureand is illustrated comprising the same source-drain metal comprising thesource and drain electrodes for TFT 130. In this embodiment source-drainmetal layer 140 typically comprises a two-tier structure comprising afirst conductive layer 142 and a cap layer 144 (also known as anetch-stop layer) comprising a second conductive material. Firstconductive layer 142 typically comprises a material having a relativelylow resistivity (e.g., less than about 10 μΩ/cm), such as molybdenum, orthe like. First conductive layer 142 is deposited in a sputteringprocess to a thickness in the range between about 100 nm and 1000 nm.Cap layer 144 is deposited over first conductive layer 142 and comprisesa material selected to exhibit the desired etch stop characteristics inthe presence of etchants used in the formation of the photosensor. Caplayer 144 comprises chromium, titanium, or the like, and is deposited ina sputter deposition process to a thickness in the range between about10 nm and 100 nm, and commonly has a thickness of about 50 nm.

In accordance with this invention, prior to patterning source-drainmetal layer 140, a photosensor island 150 is formed on source-drainmetal layer 140 such that the photosensor island substantially occupiesportions of the area of pixel 120 other than where TFT body 130 isdisposed. The formation of photosensor island 150 comprises depositing alayer of photosensitive material over source-drain metal layer 140 andthen patterning the photosensitive material layer to produce photosensorisland 150. The patterning steps typically comprise forming a mask (notshown) on the photosensitive material (such as by depositingphotoresist, exposing the photoresist in accordance with a desiredpattern, and then processing the photoresist to remove portions thereof,leaving a mask having a selected pattern corresponding to the desiredshape of photosensor island 150) and then etching the photosensormaterial with an etchant selective to the material comprising the uppersurface of the source-drain metal layer 140 (as illustrated in FIG. 1,cap layer 144) so as to remove the photosensitive material and exposethe source-drain metal layer in unmasked areas of the pixel. Thephotosensitive material comprises a-Si and typically comprises a layerof intrinsic amorphous silicon disposed between an a-Si layer doped toexhibit n type conductivity and an a-Si layer doped to exhibit p typeconductivity. The etchant used in etching the silicon formingphotosensor island 150 typically comprises either fluorine or chlorine,e.g., carbon tetrafluoride (CF₄); sulfur hexafluoride (SF₆); or hydrogenchloride gas (HCl). These etchants are selective to the materialcomprising cap layer 144 (e.g., chromium), and the presence ofsource-drain metal layer 140 having an upper surface (e.g., cap layer144) that serves as an etch-stop protects TFT body 130 (especially thea-Si components in TFT body) from attack by the etchants used in formingphotosensor island 150.

Following formation of photosensor island 150, fabrication of TFT 130 iscompleted. In the embodiment of the invention illustrated in FIGS. 1 and2, exposed portions of cap layer 144 are removed in an etching process,e.g., a wet etching process using hydrochloric acid (HCl), oralternatively a dry etching process, such as reactive ion etching withchlorine and oxygen (photosensor island 150 serving as a mask foretching the exposed cap layer material), leaving photosensor island 150disposed over the remaining portions of cap layer 144 (which forms thecontact pad for the photosensor island to the TFT) and the (intact)first conductive layer 142 of source-drain metal layer 140. Inaccordance with this invention, photosensor island 150 is disposedadjacent to the remaining portion of cap layer 144 comprising thephotosensor contact pad such that substantially no material is disposedtherebetween at any position where the photosensor island overlies thecontact electrode, thus providing a structure that reduces photodiodeleakage.

Remaining portions of source-drain metal layer 140 are next patterned toform a source electrode 137 (FIG. 2) and a drain electrode 139 for TFT130 using, for example, known photolithographic techniques. Further, tocomplete fabrication of the TFT 130, doped semiconductor layer 138 isetched, for example in a reactive ion etching process comprising HCl andSF₆ (or alternatively, either of these gasses separately), in the areabetween source electrode 137 and drain electrode 139 to form a channelregion 131 substantially disposed over gate electrode 132.

In accordance with this invention, a common passivation layer 160 isthen deposited over pixel 110 so that it is disposed adjacent to theouter surfaces of both TFT 130 and photosensor island 150. Passivationlayer 160 is substantially monolithic, that is, it comprises materialthat is substantially uniformly disposed across the pixel area in adeposition process so that there are not intervening materials betweenthe passivation material and the covered components at different pointson the array. Passivation layer 160 comprises either an inorganicdielectric material such as silicon nitride, silicon dioxide, or thelike, or alternatively an organic dielectric material, such aspolyimide. Inorganic materials such as silicon nitride or silicon oxideare typically deposited using plasma enhanced chemical vapor deposition,and such a single layer of inorganic dielectric material comprisingpassivation layer 160 typically has a thickness in the range of betweenabout 0.5 μm and 1.0 μm. Organic dielectric materials such as polyimideare typically deposited by a spin coating process to a thickness in therange between about 0.5 μm and 2.5 μm. Alternatively, as disclosed inco-pending application entitled "Photosensitive Element With Two LayerPassivation Coating", now U.S. Pat. No. 5,233,181, which is assigned tothe assignee of the present invention and incorporated herein byreference, passivation layer 160 comprises multiple layers of dielectricmaterial, for example a first layer (not shown) of inorganic dielectricmaterial such as silicon nitride and a second layer comprising anorganic dielectric material such as polyimide.

Following deposition of passivation layer 160, a common electrode 170(FIG. 3)is formed. Portions of passivation layer 160 disposed over thetop surface of photosensor island 150 are patterned, using, e.g., knownphotolithographic techniques, to form a via opening 165 to a contactarea 159 on the photodiode. Common electrode 170 is then deposited sothat it is disposed over passivation layer 160 and in via opening 165 soas to be in electrical contact with contact area 159. Common electrode170 typically comprises a substantially light transmissive conductivematerial such as indium tin oxide or the like, and is deposited in asputtering process to a thickness in the range between about 0.05 μm and0.5 μm. Formation of common electrode 170 completes the fabricationsteps necessary to form the components of photosensor array 120 (othersteps, not pertinent to the present invention, such as disposing ascintillator on the photosensor array, are necessary to completefabrication of imager wafer 100).

In one alternative embodiment of the present invention, cap layer 144 isnot removed after patterning photosensor island 150. In this embodiment,cap layer 140 remains part of source-drain metal layer 140 in theassembled device.

In another alternative embodiment of the method of this invention, aseparately-deposited metal layer having the desired low-resistancecharacteristics is used to form a separate data line (not shown) afterthe fabrication of TFT 130 is complete; in this embodiment source-drainmetal layer 140 typically comprises a single layer of conductivematerial having the desired etch stop characteristics, such as chromiumor the like. Source-drain metal layer 140 in this embodiment istypically deposited in a sputtering process to a thickness in the rangebetween about 50 nm and 500 nm. Except as noted herein, other steps inthe imager fabrication process are the same as described above.

In a further embodiment of the method of this invention, source-drainmetal layer 140 comprises a single layer of metal that is not resistantto etchants used in formation of photosensor island 140. Etch protectionfor TFT body 130 is provided by depositing source-drain metal layer 140to a selected thickness such that, after completion of formation ofphotosensor island 150, sufficient source-drain metal material remainsto be patterned to form source electrode 137 and drain electrode 139,each of these electrodes having sufficient thickness so as to provide apredetermined relatively low resistance. Typically in this embodimentsource-drain metal layer 140 comprises molybdenum deposited to athickness of between about 200 nm and 2000 nm, which thickness issufficient such that not greater than about 90% the thickness of thelayer is removed in the etching of photosensor island 150. The remainingportions of source-drain metal layer 140, when patterned to form sourceelectrode 137 and drain electrode 139 exhibit relatively low resistanceso that photosensor array noise is maintained at or below an acceptablelevel for the particular application of the array. Except as notedherein, other steps in the imager fabrication process are the same asdescribed above.

The process of the present invention thus eliminates one passivationlayer and two patterning steps as compared with the conventional arrayfabrication processes. In the conventional process, the TFT fabricationis completed prior to the formation of the photosensor islands, thusnecessitating a separate passivation layer deposited over the TFT aloneto protect it during the fabrication of the photosensor islands. Thisseparate passivation layer must be patterned to provide the necessarycontact area to source-drain metal for the contact between thephotodiode and the TFT. The second photolithographic step that isobviated by the process of the current invention is the step to make thevia contacts (not shown) at the edges of the array structure for addressline connections to contact fingers (that are coupled to contacts forthe processing circuits off of the array). In the conventional arrayfabrication techniques, the presence of the two separate passivationlayers necessitated separate masking steps to form the via contactsthrough the multiple passivation layers, whereas the single commonpassivation layer of the present invention enables one photolithographicstep to be used to form the via contacts.

As illustrated in FIG. 3, each pixel of the imager fabricated inaccordance with this invention thus comprises photosensor array 120having a single common passivation layer 160 that is disposed adjacentto (that is, in physical contact with) the outer surfaces of both TFT130 and at least some portion (excluding, that is, the contact area 159that is in contact with common electrode 170) of photosensor island 150.Further, as noted herein, the photosensor array in accordance with thepresent invention provides a photosensor island 150 in which the a-Si isdisposed in contact with the underlying photosensor bottom contact padover the entire length of the bottom contact pad, thus providing astructure that is less prone to diode leakage.

While only certain features of the invention have been illustrated anddescribed herein, many modifications and changes will occur to thoseskilled in the art. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

What is claimed is:
 1. A solid state radiation imager comprising:aphotosensor array disposed on a substrate, said photosensor arraycomprising a plurality of individually-addressable pixels, each of saidpixels comprising: a photosensor having a body comprising photosensitivematerial disposed on an electrically conductive contact pad comprising afirst conductive material; a thin film transistor (TFT) having asource-drain metal layer comprising said first conductive material: saidsource-drain metal layer being electrically coupled to and contiguouswith said photosensor contact pad; said source-drain metal layer furtherbeing electrically coupled to an address line, said TFT being disposedso as to selectively couple said photosensor to said address line; asubstantially monolithic common passivation layer disposed over said TFTand said photosensor, said passivation layer being disposed adjacent toan outer surface of said TFT and at least portions of an outer surfaceof the photosensor island; and a common electrode layer disposed oversaid common passivation layer, said common electrode being coupled tosaid photosensor at an upper surface of said photosensor body.
 2. Theradiation imager of claim 1 wherein said common passivation layercomprises an organic dielectric material.
 3. The radiation imager ofclaim 2 wherein said organic dielectric material comprises polyimide. 4.The radiation imager of claim 1 wherein said common passivation layercomprises an inorganic dielectric material.
 5. The radiation imager ofclaim 4 wherein said inorganic dielectric material is selected from thegroup consisting of silicon oxide and silicon nitride.
 6. The radiationimager of claim 1 wherein said passivation layer comprises a first and asecond stratum, one of said strata comprising inorganic material and theother of said strata comprising organic dielectric material.
 7. Theradiation imager of claim 1 wherein said photosensor island is disposedadjacent to said contact pad such that substantially no passivationlayer material is disposed therebetween at any position where saidphotosensor island overlies said contact pad.
 8. The radiation imager ofclaim 7 wherein said photosensor island contact pad comprises chromium.9. The radiation imager of claim 8 wherein the source and drain of saidTFT comprise a conductive material selected from the group consisting ofmolybdenum and titanium.
 10. The radiation imager of claim 9 whereinsaid photosensor comprises a photodiode.